Mute: Just stick a shortcut on your homescreen and it silences everything with a single press
Astro: Great file manager
Barcode Scanner: Useful little utility
Spare Parts: Some advanced tweaks that are pretty awesome
Screenshot (for rooted phones)
Light Racer: Simple but fun
Panda Firewall (think this may need root) awesome app that can block calls and SMS from blacklisted numbers
Shopsavvy: Useful shopping tool
FotMob: Great app for all footy fans
Terminal Emulator (for rooted phones): Fully featured unix terminal emulator. A must for advanced configuration and executing shell scripts
Ultimate Stopwatch: Pretty good timekeeping app
YouTube Downloader: Not available on the Android Market, have to download from a 3rd party website
WikiMobile: Great wikipedia app
Flashlight: Good for those dark spots

Also, if you use the Cyanogen Mod custom ROM series on your jailbroken phone, the Cyanogen Updater is also very good

ACV - Comic viewerAndFTP - FTP ClientConnectBot - SSH ClienthandyCalc - CalculatorLocale - Automatically set phone options based on time and location, great for turning off your Ring Tone at work, or at least changing it from Get Low to something a little more work appropriateMetal Detector - Not very useful but definitely fun to play around withPhonalyzer - Compiles various stats about your calls into easy to read graphsSwiFTP - FTP Server